In an accessory mount mechanism being releasably and adjustably assembled to the accessory mount rail of a firearm a lever-actuated clamping mechanism employs an upwardly facing clamping force adjustment member that is manually adjustable by the firearm user without the use of tools of any kind. A locking lever is pivotally mounted to a lateral platform of a mount base by means of an integral spindle shaft that projects upwardly from a cam plate that is fixed to the locking lever. The spindle shaft defines a non-circular section that is engaged by a corresponding non-circular drive receptacle of a drive washer member that is urged upwardly within an insert member of the mount base by a spring member. A manually actuated adjustment member is threaded to the spindle shaft and faces upwardly for ease of visual location and manual rotation by the user of the firearm.